1. Field of the Invention
The invention relates to a method for modeling a stratified and fractured geologic environment in order to predict better the fluid flows likely to occur through this environment.
The method according to the invention is notably suitable for the study of the hydraulic properties of fractured formations and notably the study of hydrocarbon displacements in subsurface reservoirs whose structure has been modeled.
2. Description of the Prior Art
It is convenient to use a representation of a fractured rock as a starting point to study the way fluids move therein. A fractured rock is usually translated into a geometric model in which a set of well-defined geometric objects is placed in a conventional representation. These objects, that are for example fractures, i.e. surfaces of breakage of the rock, can be schematized for example by disks, ellipses or any other geometric surface. In this approach, the geometric model is of the stochastic and discrete type. It is discrete because each fracture is represented individually therein by a geometric element. It is stochastic because the aim is not to represent a well-defined real fractured block of rock, with all the fractures that can be directly observed in the field. With this type of stochastic model, a block of rock is represented by a synthetic block reproducing certain statistical properties of the real environment. In the synthetic block, the dimensions and/or the orientations of the fractures follow the same statistical laws as those of a real block.
After the model of the environment has at first been selected, the flow of fluids is calculated by applying the laws of physics. The results of this calculation thus constitute a more or less precise approximation of the flow behaviour of these fluids in a real environment.
It is obvious that the validity of the predictions achieved by means of this combined modeling is highly dependent on the quality of the geometric model selected, i.e. the resemblance between the model and the real environment represented thereby.
Geologic observations of stratified environments show that they are often damaged by fractures in a direction quasi-perpendicular to the stratification planes or interface planes (FIG. 1), and whose ends stop at these planes. These are "diaclases", which are fractures of the rock without relative displacement of the faces of the fracture plane. A diaclase family occurs in the form of quasi-parallel and evenly spaced fracture planes. A given rock can have several diaclase families that intersect and form a network. Such diaclases also have certain geometric properties that have to be taken into account in a petroleum context:
a) In a given material, it has been observed that the density of diaclases in each stratum is proportional to the thickness thereof. This property is true among other things for a material whose strata have variable thicknesses. Thin strata are characterized by a high density of diaclases; they form therefore a most suitable passageway for fluid flows. As for the layers, they have a lower density of diaclases and they consequently are an obstacle to fluid flows. PA1 b) Interfaces between strata are more or less a considerable obstacle to the extension of diaclases. Inter-strata surfaces where the diaclases stop systematically and, conversely, others which for the most part are crossed thereby can be observed. These observations show that the displacement possibilities of the fluids through these interfaces are highly dependent on the nature thereof. An interface that does not stop the progress of diaclases does not hinder a flow. In the opposite case, it will stop fluid flow. PA1 selecting by lot the number of disks to be positioned; PA1 selecting by lot the position of these disks in the space of the model; and PA1 selecting by lot the orientation and the radius of each disk. PA1 it takes into account the variation of the fracturation densities as a function of the thickness of the strata, and PA1 provides compliance with the proportion observable in the field between fractures crossing the interfaces and fractures stopping at these interfaces.
There are well-known discrete stochastic type geometric models that are directed towards the representation of homogeneous environments. They are obtained by a method in which the magnitudes that define them are selected by lot, while complying with the statistical properties of the modeled environment. The geometric objects to be positioned in the model are for example disks. The conventional technique consists for example in:
This approach is well-suited to homogeneous environments but it is difficult to transpose to the stratified environments representing the structure of the geometry of fracture networks.